Rotatable filter for a pre-subject ct collimator having multiple filtering profiles

ABSTRACT

The present invention is directed to a method and apparatus for CT data acquisition using a rotatable pre-subject filter having more than one filtering profile to control radiation exposure to a subject. The filter has one profile used to filter radiation when the radiation source is positioned above a subject and another profile that is used to filter radiation when the radiation source is positioned at a side of the subject.

BACKGROUND OF INVENTION

[0001] The present invention relates generally to diagnostic imagingand, more particularly, to a method and apparatus of dynamicallyfiltering radiation emitted toward a subject during radiographicimaging.

[0002] Typically, in radiographic imaging systems, an x-ray source emitsx-rays toward a subject or object, such as a patient or a piece ofluggage. Hereinafter, the terms “subject” and “object” may beinterchangeably used to describe anything capable of being imaged. Thebeam, after being attenuated by the subject, impinges upon an array ofradiation detectors. The intensity of the attenuated beam radiationreceived at the detector array is typically dependent upon theattenuation of the x-rays. Each detector element of the detector arrayproduces a separate electrical signal indicative of the attenuated beamreceived by each detector element. The electrical signals aretransmitted to a data processing system for analysis which ultimatelyproduces an image.

[0003] In computed tomography (CT) imaging systems, the x-ray source andthe detector array are rotated about a gantry within an imaging planeand around the subject. X-ray sources typically include x-ray tubes,which emit the x-rays as a beam at a focal point. X-ray detectorstypically include a collimator for collimating x-ray beams received atthe detector, a scintillator for converting x-rays to light energyadjacent the collimator, and a photodiode for receiving the light energyfrom an adjacent scintillator and producing electrical signalstherefrom. Typically, each scintillator of a scintillator array convertsx-rays to light energy. Each photodiode detects the light energy andgenerates a corresponding electrical signal. The outputs of thephotodiodes are then transmitted to the data processing system for imagereconstruction.

[0004] There is increasingly a need to reduce radiation dosage projectedtoward a subject during an imaging session. It is generally well knownthat significant dose reduction may be achieved by using a “bowtie”filter to shape the intensity profile of an x-ray beam. Surface dosereductions maybe as much as 50% using a bowtie filter Generally, thebowtie filter is constructed such that the profile used to filter theradiation projected toward the subject is constant through the imagingprocess. That is, the bowtie filter is constructed to filter radiationemitted toward a subject such that the sum of the radiation path lengththrough the filter plus the path length through the subject is roughlyconstant One drawback of this construction is that a subject's shape isnot constant For example, a patient's shape changes as a function ofview angle as the x-ray tube and detector rotate around the patient andbecause the filter has a constant filtering profile at different pointsin the imaging process, portions of the subject may be over-radiatedwhereas at other times, other portions will be under-radiated As aresult a choice must be made between having under-radiated portionsduring the imaging process or increasing the dosage such that a minimumradiation dosage throughout the imaging session is maintained with someportion being over-radiated.

[0005] A number of developments have been made to address thissituation. The CT scanner may be equipped with multiple beam shapingfilters. Multiple filters, however, increases the manufacturing costs ofthe CT scanner as well as requiring additional controls to insure theappropriate filters are being used during the scan session. Anotherapproach is to dynamically control the amount of x-ray dosage emitted bythe projection source as a function of view angle. This approach,however, requires significant and complex controls to alter the amountof radiation projected toward the subject during the scan thatsignificantly increases the costs of the CT system nor takes intoaccount shape of the pre-patient filter.

[0006] Therefore, it would be desirable to design a pre-subjectcollimator having a filter that rotates and effectively changes itsfiltering and been shaping properties as a function of view angle as thex-ray tube and detector rotate around a subject.

BRIEF DESCRIPTION OF INVENTION

[0007] The present invention is a directed method and apparatus for CTdata acquisition using a rotatable pre-subject filter having multiplefiltering profiles to dynamically control radiation exposure to asubject that overcomes the aforementioned drawbacks.

[0008] Specifically, a filter is disclosed that is shaped to havevarying filtering profiles. The filter is designed to rotate about itslengthwise axis such that the filtering profile used may be varied as afunction of the view angle of the projection source.

[0009] Therefore, in accordance with one aspect of the presentinvention, a beam shaping filter for a CT imaging system includes afirst end and a second end, each of which has a circular cross-section.The filter further includes a body formed of radiation beam shapingmaterial that extends between the first end and the second end. A pairof depressions is formed in the body orthogonal from one another suchthat a first body diameter and a second body diameter are defined.

[0010] In accordance with another aspect of the present invention, a CTsystem includes a rotatable gantry having an opening to receive asubject to be scanned as well as a rotatable high frequencyelectromagnetic energy projection source configured to project a highfrequency electromagnetic energy beam toward the subject at at least twoview angles. The rotatable pre-subject collimator filter is alsoprovided and has a static shape that defines at least two filteringprofiles such that at a first view angle a first filtering profilefilters the high frequency electromagnetic energy beam and at a secondview angle a second filtering profile filters the high frequencyelectromagnetic energy beam. The CT system further includes ascintillator array having a plurality of scintillator cells wherein eachcell is configured to detect high frequency electromagnetic energypassing through the subject as well as a photodiode array opticallycoupled to the scintillator array and comprising a plurality ofphotodiodes configured to detect light output from a correspondingscintillator cell. A data acquisition system is provided and connectedto the photodiode array and configured to receive photodiode outputswhich are received by an image reconstructor that is configured toreconstruct an image of the subject.

[0011] According to another aspect of the present invention, a method ofreducing x-ray exposure during CT data acquisition includes the steps ofpositioning a subject to be scanned in a scanning bay and projectingx-rays toward the subject from an x-ray source at a first view angle.The method also includes positioning a first profile of a multi-profilefilter between x-ray source and the subject when the x-ray source isprojecting x-rays at the first view angle. The method further includesthe steps of spinning an x-ray source to a second view angle andprojecting x-rays toward the subject from the x-ray source at the secondview angle. A second profile of the multi-profile filter is thenpositioned between an x-ray source and the subject when the x-ray sourceis projecting x-rays at the second view angle.

[0012] Various other features, objects and advantages of the presentinvention will be made apparent from the following detailed descriptionand the drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0013] The drawings illustrate one preferred embodiment presentlycontemplated for carrying out the invention.

[0014] In the drawings:

[0015]FIG. 1 is a pictorial view of a CT imaging system.

[0016]FIG. 2 is a block schematic diagram of the system illustrated inFIG. 1.

[0017]FIG. 3 is a perspective view of one embodiment of a CT systemdetector array.

[0018]FIG. 4 is a perspective view of one embodiment of a detector.

[0019]FIG. 5 is illustrative of various configurations of the detectorin FIG. 4 in a four-slice mode.

[0020]FIG. 6 is a perspective view of a rotatable pre-patient collimatorfilter assembly in accordance with the present invention.

[0021]FIG. 7 is a cross-sectional view taken lengthwise along the filtershown in FIG. 6.

[0022]FIG. 8 is a cross-sectional view taken lengthwise along the filtershowing FIG. 6 and orthogonal to the cross-sectional view shown in FIG.7.

[0023]FIG. 9 is a schematic view of the pre-patient collimator filtershown in FIG. 6 connected to a motor and a bearing assembly.

[0024]FIG. 10 is a pictorial view of a CT system for use with anon-invasive package inspection system.

DETAILED DESCRIPTION

[0025] The present invention is described with respect to a radiographicimaging system such as the CT system shown in FIGS. 1-2.

[0026] However, it will be appreciated by those skilled in the art thatthe present invention is equally applicable for use with otherradiographic imaging systems utilizing a rotating projection source anda pre-subject collimator. Moreover, the present invention will bedescribed with respect to the emission and detection of x-rays. However,one skilled in the art will further appreciate, that the presentinvention is equally applicable for the emission and detection of otherhigh frequency electromagnetic energy.

[0027] Referring to FIGS. 1 and 2, a “third generation” CT imagingsystem 10 is shown as including a gantry 12. The present invention,however, is applicable with other CT systems. Gantry 12 has an x-raysource 14 that projects a beam of x-rays 16 through filter 15 toward adetector array 18 on the opposite side of the gantry 12. Detector array18 is formed by a plurality of detectors 20 which together sense theprojected x-rays that pass through a medical patient 22. Each detector20 produces an electrical signal that represents the intensity of animpinging x-ray beam and hence the attenuated beam as it passes throughthe patient 22. During a scan to acquire x-ray projection data, gantry12 and the components mounted thereon rotate about a center of rotation24.

[0028] Rotation of gantry 12 and the operation of x-ray source 14 aregoverned by a control mechanism 26 of CT system 10. Control mechanism 26includes an x-ray controller 28 that provides power and timing signalsto an x-ray source 14, a gantry motor controller 30 that controls therotational speed and position of gantry 12, and filter controller 33that controls filter 15. As will be described in greater detail below,filter 15 is rotated or caused to spin about a lengthwise axis ofrotation by motor assembly 17. A data acquisition system (DAS) 32 incontrol mechanism 26 samples analog data from detectors 20 and convertsthe data to digital signals for subsequent processing. An imagereconstructor 34 receives sampled and digitized x-ray data from DAS 32and performs high speed reconstruction. The reconstructed image isapplied as an input to a computer 36 which stores the image in a massstorage device 38.

[0029] Computer 36 also receives commands and scanning parameters froman operator via console 40 that has a keyboard. An associated cathoderay tube display 42 allows the operator to observe the reconstructedimage and other data from computer 36. The operator supplied commandsand parameters are used by computer 36 to provide control signals andinformation to DAS 32, x-ray controller 28 and gantry motor controller30. In addition, computer 36 operates a table motor controller 44 whichcontrols a motorized table 46 to position patient 22 and gantry 12.Particularly, table 46 moves portions of patient 22 through a gantryopening 48.

[0030] As shown in FIGS. 3 and 4, detector array 18 includes a pluralityof scintillators 57 forming a scintillator array 56. A collimator (notshown) is positioned above scintillator array 56 to collimate x-raybeams 16 before such beams impinge upon scintillator array 56.

[0031] In one embodiment, shown in FIG. 3, detector array 18 includes 57detectors 20, each detector 20 having an array size of 16×16. As aresult, array 18 has 16 rows and 912 columns (16×57 detectors) whichallows 16 simultaneous slices of data to be collected with each rotationof gantry 12.

[0032] Switch arrays 80 and 82, FIG. 4, are multi-dimensionalsemiconductor arrays coupled between scintillator array 56 and DAS 32.Switch arrays 80 and 82 include a plurality of field effect transistors(FET) (not shown) arranged as multi-dimensional array. The FET arrayincludes a number of electrical leads connected to each of therespective photodiodes 60 and a number of output leads electricallyconnected to DAS 32 via a flexible electrical interface 84.Particularly, about one-half of photodiode outputs are electricallyconnected to switch 80 with the other one-half of photodiode outputselectrically connected to switch 82. Additionally, a reflector layer(not shown) may be interposed between each scintillator 57 to reducelight scattering from adjacent scintillators. Each detector 20 issecured to a detector frame 77, FIG. 3, by mounting brackets 79.

[0033] Switch arrays 80 and 82 further include a decoder (not shown)that enables, disables, or combines photodiode outputs in accordancewith a desired number of slices and slice resolutions for each slice.Decoder, in one embodiment, is a decoder chip or a FET controller asknown in the art. Decoder includes a plurality of output and controllines coupled to switch arrays 80 and, 82 and DAS 32. In one embodimentdefined as a 16 slice mode, decoder enables switch arrays 80 and 82 sothat all rows of the photodiode array 52 are activated, resulting in 16simultaneous slices of data for processing by DAS 32. Of course, manyother slice combinations are possible. For example, decoder may alsoselect from other slice modes, including one, two, and four-slice modes.

[0034] As shown in FIG. 5, by transmitting the appropriate decoderinstructions, switch arrays 80 and 82 can be configured in thefour-slice mode so that the data is collected from four slices of one ormore rows of photodiode array 52. Depending upon the specificconfiguration of switch arrays 80 and 82, various combinations ofphotodiodes 60 can be enabled, disabled, or combined so that the slicethickness may consist of one, two, three, or four rows of scintillatorarray elements 57. Additional examples include, a single slice modeincluding one slice with slices ranging from 1.25 mm thick to 20 mmthick, and a two slice mode including two slices with slices rangingfrom 1.25 mm thick to 10 mm thick. Additional modes beyond thosedescribed are contemplated.

[0035] Referring now to FIG. 6, a statically shaped pre-patientcollimator filter 15 is shown in perspective as being connectable to amotor or other drive assembly 17 via a shaft 86 that is operationallyconnected to the filter 15 via end cap or plate 88. End cap 88 isdesigned to be adhesively connected to one end 90 of filter 15. Asimilarly shaped end cap or plate 92 is connected to an opposite end 94of the filter. The operational significance of the shaft and end capswill be described in greater detail with respect to FIG. 10. The shapeof the filter is static or constant, but the shape defines multiplefiltering profiles, as hereinafter described.

[0036] Still referring to FIG. 6, filter 15 has a general bowtie orhourglass shape. In this regard, each of the ends 90, 94 of filter 15has a diameter that is greater than the diameter of the body 96 thatextends between each of the ends. Body 96, however, is uniquely designedto have a depression defined by surface 98 as well as a seconddepression defined by surface 100. In the embodiment illustrated in FIG.6, each of the depressions is positioned orthogonally from one another.The orthogonal placement of the depressions illustrates only one exampleof how the body 96 of filter 15 may be constructed. As such, thedepressions may be formed in the body 96 of filter 15 so as to be moreacutely or obtusely defined with respect to one another. Further, thebody may be constructed to have multiple depressions including more thantwo depressions. As will be discussed in greater detail below, thedepressions define the shape of filter 15 such that a multitude offiltering profiles is possible.

[0037] Referring now to FIG. 7, a cross-sectional view of filter 15illustrating a first filtering profile is shown. It is readily apparentin FIG. 7 that the body 96 of filter 15 extending between ends 90 and 94includes a relatively steep depression defined by surface 100. In thisregard, a minimum body diameter is formed at the base 102 of thedepression. The filtering profile of filter 15 that results from theorientation illustrated in FIG. 7 is particularly well suited forpre-patient filtering of an x-ray beam that is being projected towardthe patient from a projection source generally positioned at a side ofthe patient, i.e. when the patient has the thickest cross-section. Thatis, since a patient generally has a thicker cross-section in thex-direction than in the y-direction, when lying on a patient table,filter 15 is designed such that the filtering profile illustrated inFIG. 7 is presented when x-rays are projected toward a thicker patientcross-section. Simply, filter 15 is designed such that x-rays receivedat surfaces 104 of filter 15 undergo greater filtering than the x-raysreceived at surfaces 100, 102. This variation in filtering powerthroughout the filter ensures that the sum of the x-ray beam lengths isuniform at the x-ray detector array interface.

[0038] Referring now to FIG. 8, another cross-sectional view of filter15 is shown that is orthogonal to the cross-sectional view illustratedin FIG. 7. In this regard, the filtering profile of filter 15 is definedby surface 98. Similar to the filtering profile illustrated in FIG. 7,the filtering profile of FIG. 8 also has a relative bowtie or hourglassshape, but the depression formed by surface 98 is less steep than thedepression illustrated in FIG. 7. In this regard, the filtering profileillustrated in FIG. 8 is particularly well suited for presentation inthe x-ray fan beam path when the x-ray source is projecting x-raystoward a patient from a position generally above the patient, i.e. whenthe patient has the thinnest cross-section. Because patients aregenerally flat and thin in the x-direction when positioned face-up on apatient table, the filtering profile illustrated in FIG. 8 is alsorelatively flat. That is, filter 15 is designed to provide less varyingfiltering across its width but less filtering of the fan beam when thefiltering profile illustrated in FIG. 8 is presented in the x-ray beampath.

[0039] Referring now to FIG. 9, a schematic representation of thepatient collimator filter 15 is shown connected to a motor or otherdrive assembly 17. As indicated previously, filter 15 is positionedwithin the gantry of the CT system and is designed to rotate with thegantry around the patient or other scan subject during data acquisition.However, filter 15 is also constructed to spin or rotate about itslengthwise axis generally referenced 106 such that more than onefiltering profile, such as those illustrated in FIGS. 7 and 8, may bepresented in the x-ray beam path as the filter rotates around thepatient. Accordingly, motor 17 is connected to an end cap or plate 88that is affixed to an end of filter 15. On an opposite end, plate 92 isconnected to the filter and also has a shaft 108 that is connected to abearing assembly 110. Motor 17 is designed to rotate shaft 86 inconjunction with a bearing assembly 110 so that filter 15 is caused tospin or rotate about lengthwise axis 106 as a function of view angle ofthe x-ray projection source. In this regard, motor 17 may be controlledby a filter controller 33, FIG. 2, such that a steeper filteringprofile, FIG. 7, is presented when the x-ray projection source is at aside of the patient. The motor may then cause filter 15 to spin orrotate about its lengthwise axis 106 such that the filtering profileillustrated in FIG. 8 is presented when the x-ray projection source isprojecting x-rays from a position above the patient. One skilled in theart would readily appreciate that an encoder or other device may beincorporated with the motor such that the rotation of the gantry and thespinning of filter 15 may be synchronized. Motor 17 may also becontrolled such that the filter 15 is caused to rotate or spin along itslengthwise axis uniquely for each scan session. That is, a pre-scan maybe undertaken whereupon a profile of the patient is acquired and used toinput data into a computer program or algorithm to control motor 17 suchthat the filtering profiles used throughout the imaging session arepresented according to the particular contour of the particular patientundergoing the imaging session as opposed to a general controlalgorithm, or synchronized with table translation. Additionally, themotor may incrementally spin or rotate the filter about its lengthwiseaxis or provide a timed and smooth continuous rotation as a function ofview angle.

[0040] Referring now to FIG. 11, package/baggage inspection system 200includes a rotatable gantry 202 having an opening 204 therein throughwhich packages or pieces of baggage may pass. The rotatable gantry 202houses a high frequency electromagnetic energy source 206, a pre-subjectcollimator filter 207, as well as a detector assembly 208. A conveyorsystem 210 is also provided and includes a conveyor belt 212 supportedby structure 214 to automatically and continuously pass packages orbaggage pieces 216 through opening 204 to be scanned. Objects 216 arefed through opening 204 by conveyor belt 212, imaging data is thenacquired, and the conveyor belt 212 removes the packages 216 fromopening 204 in a controlled and continuous manner. As a result, postalinspectors, baggage handlers, and other security personnel maynon-invasively inspect the contents of packages 216 for explosives,knives, guns, contraband, etc.

[0041] Therefore, in accordance with one embodiment of the presentinvention, a beam shaping filter for a CT imaging system includes afirst end and a second end, each of which has a circular cross-section.The filter further includes a body formed of radiation beam shapingmaterial that extends between the first end and the second end. A pairof depressions is formed in the body orthogonal from one another suchthat a first body diameter and a second body diameter are defined.

[0042] In accordance with another embodiment of the present invention, aCT system includes a rotatable gantry having an opening to receive asubject to be scanned as well as a rotatable high frequencyelectromagnetic energy projection source configured to project a highfrequency electromagnetic energy beam toward the subject at at least twoview angles. The rotatable pre-subject collimator filter is alsoprovided and has a static shape that defines at least two filteringprofiles such that at a first view angle a first filtering profilefilters the high frequency electromagnetic energy beam and at a secondview angle a second filtering profile filters the high frequencyelectromagnetic energy beam. The CT system further includes ascintillator array having a plurality of scintillator cells wherein eachcell is configured to detect high frequency electromagnetic energypassing through the subject as well as a photodiode array opticallycoupled to the scintillator array and comprising a plurality ofphotodiodes configured to detect light output from a correspondingscintillator cell. A data acquisition system is provided and connectedto the photodiode array and configured to receive photodiode outputswhich are received by an image reconstructor that is configured toreconstruct an image of the subject.

[0043] According to another embodiment of the present invention, amethod of reducing x-ray exposure during CT data acquisition includesthe steps of positioning a subject to be scanned in a scanning bay andprojecting x-rays toward the subject from an x-ray source at a firstview angle. The method also includes positioning a first profile of amulti-profile filter between x-ray source and the subject when the x-raysource is projecting x-rays at the first view angle. The method furtherincludes the steps of spinning an x-ray source to a second view angleand projecting x-rays toward the subject from the x-ray source at thesecond view angle. A second profile of the multi-profile filter is thenpositioned between an x-ray source and the subject when the x-ray sourceis projecting x-rays at the second view angle.

[0044] In another embodiment, the present invention includes apre-subject filter assembly for a CT imaging system having a detectorassembly and a high frequency electromagnetic energy projection sourceconfigured to rotate about a subject during an imaging session. Thefilter assembly includes a beam shaping filter having a generallycylindrical shaped body defining multiple filtering profiles. The filteris constructed to rotate about an axis of rotation that extends throughits body. A controller is provided to cause rotation of the filter aboutthe axis of rotation during an imaging session to dynamically filterhigh frequency electromagnetic energy projected toward the subject as afunction of view angle.

[0045] In another embodiment, a CT system includes a rotatable gantryhaving an opening to receive a subject to be scanned. A moveable highfrequency electromagnetic energy projection source projects highfrequency electromagnetic energy such as x-rays toward the subject at atleast two view angles during an imaging session. A pre-subjectcollimator includes a filter that is rotatable about itself relative toan axis of rotation extending the filter during the imaging session. Thefilter includes multiple filtering profiles. As such, the filter movescircumferentially around the subject during the imaging session but alsorotates or spins relative to itself during the imaging session such thatmultiple filtering profiles are usable during the imaging session. TheCT system further includes a scintillator array and a photodiode arraythat rotate synchronously with the filter and x-ray source during theimaging session. The scintillator and photodiode arrays collectivelyform a detector array that detects x-rays attenuated by the subjectduring the imaging session and outputs electrical signals indicative ofthe attenuation. The detector output is then input to a DAS configuredto reconstruct an image of the subject. The CT system further includes acomputer programmed to rotate the filter about the axis of rotation suchthat a first view angle a first filtering profile is presented and at asecond view angle a second filtering profile is presented.

[0046] In another embodiment of the present invention, a method ofreducing x-ray exposure during CT data acquisition includes positioninga subject to be scanned in a scanning bay and projecting x-rays towardthe subject from an x-ray source at a first view angle. A first profileof a multi-profile filter is then positioned between the x-ray sourceand the subject when the x-ray source is projecting x-rays at the firstview angle. The x-ray source is then rotated to a second view angle andprojects x-rays toward the subject therefrom. Simultaneously therewith,the multi-profile filter is caused to rotate about an axis of rotationthat extends through a length of the filter such that a second profileis positioned between the x-ray source and the subject when the x-raysource is projecting x-rays from the second view angle.

[0047] The present invention has been described in terms of thepreferred embodiment, and it is recognized that equivalents,alternatives, and modifications, aside from those expressly stated, arepossible and within the scope of the appending claims.

what is claimed is:
 1. A beam shaping filter for a CT imaging system,the filter comprising: a first end and a second end, each end having acircular cross-section; a body formed of radiation beam shaping materialand extending between the first end and the second end; and a pair ofdepressions formed in the body orthogonal from one another and defininga first body diameter and a second body diameter.
 2. The beam shapingfilter of claim 1 wherein the first body diameter defines a minimumfiltering profile and the second body diameter defines a maximumfiltering profile.
 3. The beam shaping filter of claim 2 wherein thebody further defines a number of filtering profiles between the pair ofdepressions.
 4. The beam shaping filter of claim 2 configured to rotateas a function of gantry view angle such that the first filtering profileis presented when the filter is generally at a side of a subject and thesecond filtering profile is presented when the filter is generally abovethe subject.
 5. The beam shaping filter of claim 1 configured tominimize radiation dosage to a subject as a function of x-ray sourceview angle.
 6. The beam shaping filter of claim 1 wherein the pair ofdepressions defines a pair of filtering profiles and wherein onefiltering profile is steeper than the other filtering profile.
 7. Thebeam shaping filter of claim 1 further comprising a shaft connectable toa motor at the first end and comprising a bearing assembly connected atthe second end.
 8. A CT system comprising: a rotatable gantry having anopening to receive a subject to be scanned; a rotatable high frequencyelectromagnetic energy projection source configured to project a highfrequency electromagnetic energy beam toward the subject at at least twoview angles; a rotatable pre-subject collimator filter, the filterhaving a static shape that defines at least two filtering profiles suchthat at a first view angle a first filtering profile filters the highfrequency electromagnetic energy beams and at a second view angle asecond filtering profile filters the high frequency electromagneticenergy beam; a scintillator array having a plurality of scintillatorcells wherein each cell is configured to detect high frequencyelectromagnetic energy passing through the subject; a photodiode arrayoptically coupled to the scintillator array and comprising a pluralityof photodiodes configured to detect light output from a correspondingscintillator cell; a data acquisition system (DAS) connected to thephotodiode array and configured to receive the photodiode outputs; andan image reconstructor connected to the DAS and configured toreconstruct an image of the subject from the photodiode outputs receivedby the DAS.
 9. The CT system of claim 8 wherein the filter includes abowtie filter designed to reduce high frequency electromagnetic energydosage to the subject as a function of projection source view angle. 10.The CT system of claim 8 wherein the first filtering profile is flatrelative to the second filtering profile.
 11. The CT system of claim 10wherein the filter includes a first end and a second end and a bodyextending therebetween, the body including at least a pair ofdepressions orthogonal from one another that defines a first bodydiameter and a second body diameter, respectively.
 12. The CT system ofclaim 11 wherein the first body diameter is less than the second bodydiameter such that the first filtering profile defined by the first bodydiameter is steeper than the second filtering profile defined by thesecond body diameter.
 13. The CT system of claim 8 wherein the firstview angle is orthogonal of the second view angle.
 14. The CT system ofclaim 8 incorporated into at least one of a medical imaging system and aparcel inspection apparatus.
 15. A method of reducing x-ray exposureduring CT data acquisition comprising the steps of: positioning asubject to be scanned in a scanning bay, the scanning bay defined by agantry having a bore therethrough and wherein the gantry indicates anx-ray source and a multi-profile filter designed to rotate around thesubject during an imaging session; projecting x-rays toward the subjectfrom an x-ray source at a first view angle; spinning the multi-profilefilter such that a first filtering profile is positioned between thex-ray source and the subject when the x-ray source is projecting x-raysat the first view angle; rotating the x-ray source to a second viewangle; projecting x-rays toward the subject from the x-ray source at thesecond view angle; and spinning the multi-profile filter such that asecond filtering profile is between the x-ray source and the subjectwhen the x-ray source is projecting x-rays at the second view angle. 16.The method of claim 15 wherein the first view angle is orthogonal to thesecond view angle.
 17. The method of claim 15 wherein the second profileis steeper than the first profile.
 18. The method of claim 17 whereinthe first view angle is relatively above the subject and the second viewangle is at a relative side of the subject.
 19. The method of claim 15further comprising the step of rotating the filter synchronously withrotation of the x-ray source.
 20. The method of claim 15 wherein thefilter includes a beam shaping bowtie filter.